As semiconductor manufacturing advances to ultra-large scale integration (ULSI), the devices on semiconductor wafers must be shrunk to sub-micron dimensions and the circuit density must be increased to several million transistors per die. In order to accomplish this high device packing density, smaller and smaller feature sizes are required. This may include the width and spaces between device interconnect lines and the surface geometry such as contact openings and the like.
The requirement of small feature sizes with close spacing between adjacent features requires high resolution photolithographic processes. In general, photolithography utilizes a beam of light, such as ultraviolet (UV) radiation, to transfer a pattern from a photolithographic reticle pattern onto a photoresist coating through an imaging lens. Reticles are generally made of laminar transparent quartz. The reticle pattern includes opaque regions (i.e., regions having a thin layer of chromium plating), as well as transparent regions. Were it not for the diffraction phenomenon, and were all light rays passing through the reticle parallel to one another, the pattern on the reticle would be transmitted exactly to the photoresist coating. However, when the dimensions of the opaque and transparent reticle regions are near the wavelength of the light utilized to project the image, diffraction becomes a significant problem. Regions on the photoresist that should be dark are illuminated with diffracted light rays.
One technique currently being investigated for mitigating the diffraction effect, so as to improve the resolution of the photolithograhic process, is known as phase shift lithography. In phase shift lithography, diffracted light rays from adjacent transparent regions of the reticle are made to cancel one another, thus eliminating the diffraction effect and improving the resolution and depth of optical images projected onto a target. The cancellation of diffracted rays from adjacent transparent regions is effected by adjusting the light path through the various transparent reticles such that light passing through any transparent region is 180 degrees out of phase with the light passing through any adjacent transparent region. Thus, when light rays are diffracted from two neighboring transparent regions of the reticle, they cancel one another when they coincide at some point below the intervening opaque region. The mathematics employed in the construction of a phase-shifting reticle are well known in the art, and will not be discussed herein.
One of the most common and successful techniques for fabricating a phase-shifting reticle is to take a conventional reticle consisting of a uniformly thick quartz layer on which a chromium layer has been patterned to produce a patten of opaque and transparent regions, mask every other transparent region with photoresist, and then subject the photoresist masked reticle to a plasma etch until the unmasked transparent regions are relieved to an extent that, when the photoresist mask is removed, rays of light from the coherent source used for the photolithographic exposure process will pass through the unetched transparent regions and exit the reticle one-half wavelength behind rays of light from the same coherent source that pass through neighboring etched transparent regions. Although such a process works acceptably in principle, the plasma etch damages the optical characteristics of the quartz so that transmittance through the etched transparent regions is reduced as compared to the transmittance through unetched transparent regions. The result is somewhat less than adequate cancellation of the neighboring diffraction patterns.